Altimeter testing device and methods

ABSTRACT

Devices and methods for testing altimeters are provided. A radio-frequency (RF) signal may be received from an altimeter and passed through an RF delay module to delay the RF signal. The delayed RF signal may be converted to an optical signal, which may be passed through an optical delay module to delay the optical signal. The system tests the accuracy of the altimeter based on the combined RF signal delay and optical signal delay.

TECHNICAL FIELD

The present disclosure relates to altimeters and, more particularly, toa device and method for testing the accuracy of altimeters.

BACKGROUND

Radio altimeters are generally used in aircraft to determine an altitudeof the aircraft. FIG. 1 illustrates an exemplary radio altimeter 10having a processor 12 and a memory 14. The memory 14 includesinstructions that, when executed by the processor 12, cause atransmission antenna 16 to transmit a signal 18 to the ground 20. Thetransmitted signal 18 is reflected off the ground 20 and a return signal22 is detected by a receiving antenna 24. The altimeter 10 includes aclock 26 that records a delay between the transmission of the signal 18and the reception of the return signal 22. The processor 12 utilizesthis delay to determine an altitude of the aircraft based on the signaltraveling at the speed of light.

Generally, altimeters are tested using synthesized delay responses to atransmission signal. That is, while the altimeter (and subsequently theaircraft) is on the ground, a testing device simulates a delayed signalto the altimeter. Unfortunately, these testing devices are signalwaveform dependent and are not operable on all altimeters. As such, notall testing devices provide traceable measurements. A need remains foran altimeter testing device with a physical delay that is waveformagnostic.

SUMMARY

The present disclosure includes one or more of the features recited inthe appended claims and/or the following features which, alone or in anycombination, may comprise patentable subject matter.

In one embodiment, a method of testing an altimeter is provided. Themethod being performed by a test device and comprising: inputting a testaltitude; receiving a radio-frequency (RF) signal from the altimeter;passing the received RF signal through at least one delay module todelay the RF signal by a delay corresponding to the test altitude; andtransmitting the delayed RF signal to the altimeter.

In another embodiment, a device for testing an altimeter is provided.The device comprises: an input device to input a test altitude; an inputto receive a radio-frequency (RF) signal from the altimeter; at leastone delay module to delay the RF signal by a delay corresponding to thetest altitude; and an output for transmitting the delayed RF signal tothe altimeter.

BRIEF DESCRIPTION OF THE DRAWINGS

The concepts described in the present disclosure are illustrated by wayof example and not by way of limitation in the accompanying figures. Forsimplicity and clarity of illustration, elements illustrated in thefigures are not necessarily drawn to scale. For example, the dimensionsof some elements may be exaggerated relative to other elements forclarity. Further, where considered appropriate, reference labels havebeen repeated among the figures to indicate corresponding or analogouselements. The detailed description particularly refers to theaccompanying figures in which:

FIG. 1 is a schematic diagram of a prior art altimeter transmitting asignal to ground and receiving a return signal;

FIG. 2 is a schematic diagram of an example altimeter testing devicehaving an RF delay module in series with an optical delay moduleconstructed in accordance with the disclosed principles; and

FIG. 3 is a flowchart of an example method of operating the altimetertesting device shown in FIG. 2 in accordance with the disclosedprinciples to test an altimeter such as e.g., the altimeter shown inFIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific embodiments thereof havebeen shown by way of example in the figures and will be described hereinin detail. It should be understood, however, that there is no intent tolimit the concepts of the present disclosure to the particular formsdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives consistent with the presentdisclosure and the appended claims.

References in the specification to “one embodiment,” “an embodiment,”“an illustrative embodiment,” etc., indicate that the embodimentdescribed may include a particular feature, structure, orcharacteristic, but every embodiment may or may not necessarily includethat particular feature, structure, or characteristic. Moreover, suchphrases are not necessarily referring to the same embodiment. Further,when a particular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to effect such feature, structure,or characteristic in connection with other embodiments whether or notexplicitly described.

In the drawings, some structural or method features may be shown inspecific arrangements and/or orderings. However, it should beappreciated that such specific arrangements and/or orderings may not berequired. Rather, in some embodiments, such features may be arranged ina different manner and/or order than shown in the illustrative figures.Additionally, the inclusion of a structural or method feature in aparticular figure is not meant to imply that such feature is required inall embodiments and, in some embodiments, may not be included or may becombined with other features.

Referring to FIG. 2, an example altimeter testing device 100 inaccordance with the disclosed principles may include a control panel 102having an antenna input 106 and an antenna output 108. The antenna input106 may be configured to couple to the transmission antenna 16 of thealtimeter 10 illustrated in FIG. 1 to receive the transmitted signal 18from the altimeter 10. The output 108 may be configured to couple to thereceiving antenna 24 of the altimeter 10 illustrated in FIG. 1 to outputa return signal from the testing device 100 to the altimeter 10. Thetesting device 100 may be configured to simulate a predeterminedaltitude of a signal transmitted from the altimeter. For example, thetesting device 100 may be set to simulate an altitude of 1000 feet asdiscussed in more detail below. During the test, the altimeter 10transmits a signal 18 to the testing device 100. The testing device 100may pass the signal 18 through a predetermined amount of cable thatcorrelates to/simulates an altitude of 1000 feet (e.g., 2000 feet ofcable) and transmits a return signal 22 to the altimeter 10. If thealtimeter 10 is functioning properly, the altimeter 10 will detect analtitude of 1000 feet based on the delay between the transmission of thesignal 18 and the reception of the return signal 22, with the delaybeing caused by the signal passing through predetermined amount of cablethat correlates to/simulates an altitude of 1000 feet (e.g., 2000 feetof cable). Notably, the testing device 100 performs several conversionsof the signal 18, as described in more detail below. Additionally, thetesting device 100 may test any predetermined altitude between 10 feetand 10,230 feet in increments of 10, as described in more detail below.

The control panel 102 may include two ports 110, 112 that are configuredto perform voltage standing wave ratio (VSWR) testing, which is aseparate function from the altimeter accuracy test described herein. Forexample, VSWR testing may be used to determine cable loss and or theimpedance of coaxial cables and antennas used in the radio altimetersystem.

A switch 120 may be provided on the control panel 102 to turn thetesting device 100 on and off. The switch 120 is coupled to a powersupply 122 that powers the testing device 100. In the illustrativeembodiment, the power supply 122 is powered by a battery pack 124 suchas e.g., a lithium ion battery. In another embodiment, the power supply122 may include a plug to retrieve power from an outlet in the aircraftor an outlet independent of the aircraft. The power supply 122 powers abackplane circuit board 126 that is coupled to a controller 128. Thecontroller 128 includes a processor (not shown), memory (not shown), andother necessary components to carry out instructions that may be used tooperate the testing device 100. In the illustrated embodiment, thecontroller 128 is coupled to a rear input/output (I/O) module 130 and atouchscreen display 132 that enable a user to operate the testing device100. For example, the touchscreen display 132 may include inputs (notshown) that enable the user to select a predetermined altitude to testand to initiate the test. As will be appreciated, the touchscreendisplay 132 may include other inputs and display features that enablethe user to operate the testing device 100. The backplane circuit board126 may also be coupled to a digital module 134 and a radio-frequency(RF) module 136 (via a ribbon connection to the digital module 134). Themodules 134 and 136 may carryout instructions from the controller 128 tomodify and alter RF signals and digital signals described herein.

In operation, the antenna input 106 passes the transmitted signal 18through an attenuator 140, which may reduce the power of the signal 18without appreciably distorting the waveform of the signal 18. The signal18, with reduced power, is transmitted to an RF delay module 150 todelay the RF signal in accordance with the disclosed principles. In theRF delay module 150, the signal may be routed through a bi-directionalcoupler 144. A switch 142 may be opened or closed to select either theRF module 136 for calibration, or the altimeter 10 to run the altimetertest. The RF signal may then be passed to a plurality of coaxial cablecoils 152 designed to correlates to/simulate various altitudes, andtheir resultant delays, when switched into the signal's path. In theillustrated example, the coaxial cable coils 152 include a first coil154 correlating to/simulating an altitude of 10 feet, a second coil 156correlating to/simulating an altitude of 20 feet, and a third coil 158correlating to/simulating an altitude of 40 feet. The first coil 154 iscoupled to a first switch 160, the second coil 156 is coupled to asecond switch 162, and the third coil is coupled to a third switch 164.In one embodiment, the length of the cable can be described by thefollowing equation, referred to herein as “Equation (1)”:

Length=2*h*v  (1)

Where h is the simulated height (i.e., altitude) and v is the velocityfactor of the medium (e.g., coaxial cable). It is known that thevelocity factor for an RF cable and a fiber optic cable (discussedbelow) are similar at approximately 0.68.

In operation of the illustrated device 100, the controller 128 opens andcloses the switches 160, 162, and 164 (via the RF module 136) based onthe predetermined altitude being tested (e.g., as set by the user viathe touchscreen display 132). For example, if the predetermined altituderequires that the signal pass through 50 feet of cable, the first switch160 and third switch 164 are closed by the controller 128 to pass thesignal through the first coil 154 and the third coil 158 to delay thesignal over 50 feet. In some embodiments, all of the switches 160, 162,164 are opened and the signal is passed through the RF delay module 150without being delayed.

In operation, the RF signal is then processed in a first stage levelcontrol 170 that in the illustrated example includes an amplifier 172and an attenuator 174, which may be used to simulate the path loss ofthe RF signal in free space at the simulated altitude. If the RF signalis fully delayed, for example if the signal is delayed by apredetermined altitude of 50 feet, a pair of switches 180 and 182 areset to a position to route the signal to a second stage level control190. The second stage level control 190 may include a pair of amplifiers192, 194 and an attenuator 196 between the amplifiers 192, 194, whichalso may be used to simulate the path loss of the RF signal in freespace at the simulated altitude. The signal is then output through theantenna output 108 to the altimeter 10 to determine whether thealtimeter 10 measures 50 feet. Although the RF delay module 150 isdescribed with respect to delaying the signal a predetermined altitudeof 50 feet, it will be appreciated that the RF delay module 150 maydelay the signal to any altitude between 10 feet and 70 feet inincrements of 10 feet based on the settings of the switches 160, 162,164.

If the predetermined altitude requires a delay of 80 feet or greater(e.g., as set by the user via the touchscreen display 132), switch 180is set to a position to route the signal to a laser diode 200. Switch182 is set to a position to connect the input of the second stage levelcontrol 190 to an output of a photodiode 290 (discussed below). Thelaser diode 200 converts the RF signal to an optical signal, which istransmitted to an optical delay module 210 by the laser diode 200. Theoptical delay module 210 includes a plurality of fiber optic coils 212designed to correlate to/simulate various altitudes, and their resultantdelays, when switched into the optical signal's path. In the illustratedembodiment, the coils 212 include a first coil 220 correlatingto/simulating an altitude of 80 feet, a second coil 222 correlatingto/simulating an altitude of 160 feet, a third coil 224 correlatingto/simulating an altitude of 320 feet, a fourth coil 226 correlatingto/simulating an altitude of 640 feet, a fifth coil 228 correlatingto/simulating an altitude of 1280 feet, a sixth coil 230 correlatingto/simulating an altitude of 2560 feet, and a seventh coil 232correlating to/simulating an altitude of 5120 feet. In one embodiment,the length of the fiber optic coils may be determined using Equation (1)described above.

A first fiber optic switch 250 is coupled to the first coil 220, asecond fiber optic switch 252 is coupled to the second coil 222, a thirdfiber optic switch 254 is coupled to the third coil 224, a fourth fiberoptic switch 256 is coupled to the fourth coil 226, a fifth fiber opticswitch 258 is coupled to the fifth coil 228, a sixth fiber optic switch262 is coupled to the sixth coil 230, and a seventh fiber optic switch260 is coupled to the seventh coil 232.

In operation, the controller 128 may selectively open and close thefiber optic switches 250, 252, 254, 256, 258, 260, 262 (via the digitalmodule 134, which is connected to the optical delay module 210 at aninput/output expander 280) based on the predetermined altitude beingtested (e.g., as set by the user via the touchscreen display 132). Forexample, if the predetermined altitude being tested is 880 feet, thecontroller 128 closes the first fiber optic switch 250 to pass theoptical signal through a simulated 80 feet of altitude/delay, closes thesecond fiber optic switch 252 to pass the optical signal through anadditional simulated 160 feet of altitude/delay, and closes the fourthfiber optic switch 256 to pass the optic signal though another simulated640 feet of altitude/delay, so that the optical signal passes through atotal of 880 feet of altitude/delay.

After being delayed, the optical signal is converted by the photodiode290 back into an RF signal. The resulting RF signal is passed throughthe second stage level control 190 in the RF delay module 150. Thesignal is then output through the antenna output 108 to the altimeter 10to determine whether the altimeter 10 measures 880 feet.

Although the optical delay module 210 is described with respect toproviding a delay of 880 feet, it will be appreciated that the opticaldelay module 210 can provide any delay between 80 feet and 10,160 feet.It will also be appreciated that for some predetermined altitudes, boththe RF delay module 150 and the optical delay module 210 may be utilizedto delay the signal 18. For example, to accommodate a delay for testing910 feet of predetermined altitude, the first fiber optic coil 220(simulating 80 feet), the second fiber optic coil 222 (simulating 160feet), and the fourth fiber optic coil 226 (simulating 640 feet) areused with the first coaxial coil 154 (simulating 10 feet) and the secondcoaxial coil 156 (simulating 20 feet).

Referring now to FIG. 3, an example of a test method 300 that may beperformed by the testing device 100 is now described. In particular, thetest device 100 may use method 300 to test the accuracy of an altimeter,which is described as being performed for altimeter 10 of FIG. 1. In oneembodiment, the method 300 is performed by the controller 128 of thetest device 100. To perform the method 300, the altimeter's antenna 16is coupled to the input 106 of test device 100 and the altimeter'santenna 24 is coupled to the output 108 of test device 100.

At block 302, the controller 128 inputs a test altitude. In theillustrated embodiment, an operator selects the predetermined testaltitude by inputting a test altitude into the touchscreen display 132.The test altitude may be selected in increments of 10 feet, from 10 feetup to 10,230 feet. The controller 128 then determines which coaxialcoils 152 and/or fiber optic cables 212 are required to delay thealtimeter's transmitted signal 18 for the predetermined altitude. Forexample, if a delay of 10 feet, 20 feet, and/or 40 feet is required toadd up to the predetermined altitude, the controller 128 selects thenecessary coaxial coils 152 at block 304. If a delay of 80 feet, 160feet, 320 feet, 640 feet, 1280 feet, 2560 feet, and/or 5120 feet isrequired to add up to the predetermined altitude, the controller 128selects the necessary fiber optic coils 212 at block 306. In someembodiments, the controller 128 may only select coaxial coils 512 (atblock 304) based on the input test altitude. In some embodiments, thecontroller 128 may only select fiber optical coils 212 (at block 306)based on the input test altitude. In some embodiments, the controller128 may selects both coaxial coils 512 (at block 304) and fiber opticcoils 212 (at block 306).

At block 308, the RF signal received from the altimeter 10 is sentthrough the selected coaxial coils 152 and/or fiber optic coils 212 ofthe testing device 100 (as described in detail above with respect toFIG. 2). The selected coaxial coils 152 and/or fiber optic coils 212delay the signal for a time that corresponds to the natural delay forthe predetermined altitude. During the testing, various components inthe testing device 100 (e.g., first stage level control 170 and secondstage level control 190) may alter the signal to account for naturalamounts of noise and feedback. At step 310, a return signal 22 is sentto the altimeter 10 after the input signal 18 is delayed based on theinput test altitude and in accordance with the disclosed principles.

The altimeter 10 may then measure the delay time of the return signal 22it receives and may indicate a measured signal distance. The operatormay then compare the measured distance to the predetermined altitude todetermine whether the altimeter 10 is accurately measuring altitude.

While certain illustrative embodiments have been described in detail inthe figures and the foregoing description, such an illustration anddescription is to be considered as exemplary and not restrictive incharacter, it being understood that only illustrative embodiments havebeen shown and described and that all changes and modifications thatcome within the spirit of the disclosure are desired to be protected.There are a plurality of advantages of the present disclosure arisingfrom the various features of the methods, systems, and articlesdescribed herein. It will be noted that alternative embodiments of themethods, systems, and articles of the present disclosure may not includeall of the features described yet still benefit from at least some ofthe advantages of such features. Those of ordinary skill in the art mayreadily devise their own implementations of the methods, systems, andarticles that incorporate one or more of the features of the presentdisclosure.

1. A method of testing an altimeter, the method being performed by atest device and comprising: inputting a test altitude; receiving aradio-frequency (RF) signal from the altimeter; passing the received RFsignal through at least one delay module to delay the RF signal by adelay corresponding to the test altitude; and transmitting the delayedRF signal to the altimeter.
 2. The method of claim 1, wherein passingthe received RF signal through the at least one delay module comprises:passing the received RF signal through an RF delay module to delay theRF signal by a first delay; converting the RF signal delayed by thefirst delay to an optical signal; passing the optical signal through anoptical delay module to delay the optical signal by a second delay; andconverting the optical signal delayed by the second delay to a return RFsignal to generate the delayed RF signal sent to the altimeter.
 3. Themethod of claim 2, wherein passing the received RF signal through the RFdelay module comprises passing the received RF signal through one ormore of a plurality of coaxial cable coils of the RF delay module. 4.The method of claim 3, wherein a length of each of the plurality ofcoaxial cable coils correlates to a respective predetermined delay forthe RF signal.
 5. The method of claim 2, wherein passing the opticalsignal through the optical delay module comprises passing the opticalsignal through one or more of a plurality of fiber optic cable coils. 6.The method of claim 5, wherein each of the plurality of fiber opticcable coils includes a different length of fiber optic cable and alength of each of the plurality of fiber optic cable coils correlates toa respective predetermined delay for the optical signal.
 7. The methodof claim 2, wherein passing the RF signal through the RF delay modulecomprises attenuating the RF signal.
 8. The method of claim 1, furthercomprising passing the received RF signal through at least one componentthat correlates to path loss of the received RF signal at the testaltitude.
 9. A device for testing an altimeter, comprising: an inputdevice to input a test altitude; an input to receive a radio-frequency(RF) signal from the altimeter; at least one delay module to delay theRF signal by a delay corresponding to the test altitude; and an outputfor transmitting the delayed RF signal to the altimeter.
 10. The deviceof claim 9, wherein the at least one delay module comprises: an RF delaymodule to delay the received RF signal by a first delay; a first signalconverter to convert the RF signal delayed by the first delay to anoptical signal; an optical delay module to delay the optical signal by asecond delay; and a second signal converter to convert the opticalsignal delayed by the second delay to a return RF signal to generate thedelayed RF signal sent to the altimeter.
 11. The device of claim 10,wherein the RF delay module comprises: a plurality of coaxial cablecoils, each including a different length of coaxial cable; and aplurality of RF switches coupled across the coaxial cables and eachbeing operable to selectively couple a corresponding one of theplurality of coaxial cable coils to a signal path of the received RFsignal through the RF delay module.
 12. The device of claim 11, whereinthe length of each of the plurality of coaxial cable coils correlates toa respective predetermined delay in the RF signal.
 13. The device ofclaim 11, wherein: a first coaxial cable of the plurality of coaxialcable coils has a length that correlates to an altitude of 10 feet, asecond coaxial cable of the plurality of coaxial cable coils has alength that correlates to an altitude of 20 feet, and a third coaxialcable of the plurality of coaxial cable coils has a length thatcorrelates to an altitude of 40 feet.
 14. The device of claim 10,wherein the optical delay module comprises: a plurality of fiber opticcable coils, each including a different length of optical fiber; and aplurality of optical switches coupled across the fiber optic cable coilsand each being operable to selectively couple a corresponding one of theplurality of fiber optic cable coils to a signal path of the opticalsignal through the optical delay module.
 15. The device of claim 14,wherein the length of each of the plurality of fiber optic cablescorrelates to a respective delay in the optical signal.
 16. The deviceof claim 14, wherein: a first fiber optic cable of the plurality offiber optic cable coils has a length that correlates to an altitude of80 feet, a second fiber optic cable of the plurality of fiber opticcable coils has a length that correlates to an altitude of 160 feet, athird fiber optic cable of the plurality of fiber optic cable coils hasa length that correlates to an altitude of 320 feet, a fourth fiberoptic cable of the plurality of fiber optic cable coils has a lengththat correlates to an altitude of 640 feet, a fifth fiber optic cable ofthe plurality of fiber optic cable coils has a length that correlates toan altitude of 1280 feet, a sixth fiber optic cable of the plurality offiber optic cable coils has a length that correlates to an altitude of2560 feet, and a seventh fiber optic cable of the plurality of fiberoptic cable coils has a length that correlates to an altitude of 5120feet.
 17. The device of claim 10, wherein the RF delay module includes adigital attenuator to attenuate the RF signal.
 18. The device of claim9, further comprising at least one component that correlates to pathloss of the received RF signal at the test altitude